Hydroformylation reactions involve the preparation of oxygenated organic compounds by the reaction of synthesis gas (carbon monoxide and hydrogen) with carbon compounds containing olefinic unsaturation (hereinafter “olefinic material”). The reaction is generally performed in the presence of a hydroformylation catalyst such as cobalt or rhodium, and results in the formation of a product comprising an aldehyde which has one more carbon atom in its molecular structure than the starting olefinic feedstock. By way of example, higher alcohols useful as intermediates in the manufacture of plasticizers, detergents, solvents, synthetic lubricants, and the like, are produced commercially in the so-called Oxo Process (i.e., transition metal catalyzed hydroformylation) by conversion of C3 or higher olefin fractions (typically C5–C12) to an aldehyde-containing oxonation product having one additional carbon atom (e.g., C6–C13). Hydrogenation and distillation yields the respective alcohols, or the aldehydes may instead be further oxidized to the respective acids.
The Oxo Process to convert olefinic material to aldehydes generally proceeds through three basic stages as explained below by specific reference to a catalyst comprising cobalt.
In the first stage, or oxonation reaction, the olefinic material and the proper proportions of CO and H2 are reacted in the presence of a cobalt-containing carbonylation catalyst to give a product comprising predominantly aldehydes containing one more carbon atom than the reacted olefin. Typically, alcohols, paraffins, acetals, and other species are also produced in the hydroformylation reaction. The catalyst can be supplied to this section by numerous methods known in the art, such as by injecting cobalt acetate (or cobalt formate) directly or by supplying cobalt from a precarbonylation stage or catalyst makeup stage in the form of a cobalt anion (Co−1) or organically soluble form of Co+2, such as cobalt naphthenate, oleate, or cobalt oxides.
The oxygenated organic mixture from the oxonation (or oxo) reactor(s), which typically contains various salts and molecular complexes of the metal from the catalyst (i.e., the “metal values”) as well as the aldehydes, alcohols, acetals and other species, referred to as the crude aldehyde or crude hydroformylation mixture, is treated in a second stage, the demetalling stage. In the demetalling stage, typically a reaction is caused to separate the metal values from the aldehyde, such as by injecting dilute acetic acid. The crude hydroformylation mixture separates into phases with the organic phase comprising the desired aldehyde separated from the aqueous phase comprising cobalt acetate. The organic phase is sent to other unit operations downstream to be converted to the desired final product.
In the third stage of the Oxo Process the metal values removed in the second stage are worked up in a way that they can be reused in the oxonation section. There are several ways taught in the prior art to work up this catalyst. For example, one way is to convert the aqueous metal salt to an organically miscible compound such as cobalt naphthenate, and inject it directly into the oxonation reactor(s). Another way is to subject the aqueous salt solution in the presence of an organic solvent to high pressure synthesis gas, converting it to active carbonyl, and delivering it to the oxonation section via extraction, stripping or the like. It would be ideal if all of the cobalt is recovered and eventually passed in the proper form to the first stage described above.
These aforementioned three process stages may occur in more or less than three distinct vessels and numerous variations and improvements, including adding to, deleting from, or combining these stages, have been proposed over the years with various degrees of success.
U.S. Pat. No. 2,816,933 observes that the most direct method of utilization of cobalt acetate consists of recycling directly the aqueous cobalt stream from the demetalling stage to the primary aldehyde synthesis zone of the oxonation reaction stage. One problem with such a scheme is it introduces considerable quantities of water in to the reactor. Excess water substantially decreases the olefin conversion and may result in reactor flooding and complete loss of reaction. Instead, the patent teaches that after injection of sufficient acetic acid to combine with all the cobalt present in the demetalling stage, the entire mixture, including crude product, is allowed to separate into aldehyde and aqueous phases in a settler. After sufficient time, the lower aqueous phase containing cobalt acetate is passed to an extraction vessel where the cobalt salt is converted into oil soluble form and finally after numerous additional steps is used to supply a portion of the catalyst requirements for the oxonation reaction. Such a procedure is complex and inefficient and adds to the operating cost of the process. In addition, acetic acid is highly soluble in the organic phase and without additional treatment downstream from the demetalling stage, too much acetic acid passes with the aldehyde to the hydrogenation (or hydro) stage. Such “additional treatment”, for instance washing with fresh water, is economically and environmentally unattractive.
Numerous other inventions directed to the more efficient use of acetic acid are taught, for instance, in U.S. Pat. Nos. 2,638,485; 2,744,936; 2,754,332; 2,757,204; 2,757,206; 2,768,974; 2,812,356; and 3,055,942.
A major improvement in the oxo process is taught in U.S. Pat. No. 4,625,067. The patent describes recovery of cobalt values by contacting the crude hydroformylation product with a stripping gas to entrain volatile cobalt compounds, in the presence of water or aqueous acid (“Cobalt Flash Process”). A large portion of cobalt values in the form of cobalt carbonyl compounds absorbed into crude aldehyde product is taken overhead in the stripper reactor(s) and returned to the oxo reactor(s) by adsorption into the olefin feed stream. The partially decobalted crude product is then passed to the demetalling reaction and contacted with aqueous acid as in the prior art Oxo Processes. In the Cobalt Flash Process the cobalt-containing aqueous phase is separated and concentrated in an evaporator, while the decobalted organic phase containing crude aldehyde product (or oxo product) is passed to the hydrogenation reaction in the case where alcohol is the desired product or to further oxidation in the case where acid is the desired product.
Here again numerous improvements on the Cobalt Flash Process have been proposed, such as in U.S. Pat. Nos. 5,235,112; 5,237,104; 5,237,105; 5,336,473; 5,410,090; 5,457,240; WO 93/24437; and WO 93/24436.
In the Cobalt Flash Process as currently practiced, after the aqueous phase comprising cobalt formate is separated from the organic phase comprising oxo product in the demetalling reaction, the aqueous phase is passed to an evaporator where cobalt formate is concentrated before being passed to a preformer wherein the aqueous cobalt salt (Co+2) is converted to oil soluble Co−1 by reaction with carbon monoxide and hydrogen in the presence of an oil phase (commonly an aldehyde or alcohol, such as the product of the oxonation reaction). The cobalt is then passed into the stripper reactors and/or oxonation reactors to supplement the cobalt recovered in the strippers. Additional fresh cobalt catalyst is typically necessary and is added as, for instance, cobalt acetate.
Although the prior art suggests that acetic acid, propionic acid, and the like, may be used in the demetalling stage, numerous patents state that formic acid is preferred (such as the aforementioned U.S. '067 at col. 5, line 55+; U.S. '112 at col. 7, line 55+; U.S. '240 at col. 8, line 15; and U.S. '473 at col. 7, line 63+).
At least part of the reason for the preference of the lower molecular weight organic acid is the lower solubility of formic acid relative to acetic acid in the aldehyde product-containing oil phase separated from the cobalt values in the demetalling reaction. The acetic acid is difficult to remove from the desired product and moreover forms acetate esters with alcohols that are still more difficult to remove.
Moreover, too much acetic acid is lost in the evaporator used to concentrate the cobalt values after the demetalling reaction. Significantly more acetic acid is lost relative in this stage relative to formic acid. This is because acetic acid has the curious property of being more volatile than its lower molecular weight congener, formic acid, at least at low concentrations typically used in the Cobalt Flash Process.
Another reason for the preference of formic acid in practice is that formic acid is a natural by-product of the hydroformylation reaction; thus the loss of formic acid in the demetalling reaction (to aldehyde product contamination) is mitigated to some extent by this by-product production.
However, provided the aforementioned drawbacks could be solved, the use of acetic acid in place of formic acid in the Cobalt Flash Process offers certain benefits relative to formic acid, such as:
formic acid is corrosive to the process equipment, leading to high maintenance costs and low service factors. The replacement of the reactors and other units in the system is extremely expensive. Also, the high corrosive rate results in buildup of corrosive metals in the water, requiring frequent purging and hence loss of cobalt and an addition to the environmental load of the process, which is expensive to mitigate;
the solubility of cobalt in aqueous acetic acid is significantly higher than in aqueous formic acid, thus allowing for greater product throughput, provided, again, that the aforementioned problems could be solved;
the rate of preforming is higher with cobalt acetate as compared with cobalt formate, thus allowing higher flow rate through the preformer.
Accordingly, it would be advantageous to overcome the aforementioned drawbacks associated with the use of acetic acid in the Cobalt Flash Process and replace the formic acid currently used.